What makes certain grades of oil shear more?

[dsmith41]

In my truck, I ran M1 5w30 and my UOA showed that it sheared down to a 20 weight by the time I had changed the oil. Then, I ran M1 0w30 and the oil stayed within spec for 30 weight oil even after the same number of miles on the oil which was roughly 6500 miles. The additive packages appear to very similar with both oils.

Also, I have heard on here by different people that the 15w40 Rotella is more shear resistant than the 5w40 version of Rotella. So what determines whether or not an oil can hold its grade?

 

[tomcat27]

before synthetics, oil was actually the lower viscosity with Viscosity Improvers added to make it behave like the upper number. but the wider the range made it less stable. now, with oil containing combinations of different base stocks and different additive packages, its a different story... somewhat

 

[Captain_Klink]

it's possible the 0w30 could shear down, and then thicken as it degenerated further.